To help scientists and engineers follow the action 154 million miles away, the trajectory of the Mars Science Laboratory was set up to make sure the rover's descent to the surface of the red planet occurs within view of three orbiting satellites.

NASA's Mars Odyssey and the Mars Reconnaissance Orbiter, along with the European Space Agency's Mars Express satellite, will capture telemetry from the Mars Science Laboratory as the spacecraft makes its nail-biting seven-minute plunge to the floor of Gale Crater overnight Sunday.

NASA's Mars Odyssey spacecraft, in orbit around the red planet since 2003, will relay telemetry from Mars Science Laboratory rover during the craft's descent to the surface.
NASA

But Odyssey is the only one of the three capable of "bent pipe" realtime relay, sending the UHF telemetry directly back to Earth as the descent proceeds to give anxious engineers what amounts to continuous play-by-play updates, including confirmation of landing.

Touchdown is expected at 1:17 a.m. EDT (GMT-4) on Aug. 6, but radio signals confirming the event will take 13.8 minutes to cover the 154 million miles between Earth and Mars, arriving around 1:31 a.m. "Earth-received time."

Unexpected problems with Odyssey's attitude control system in June changed the satellite's orbit slightly, putting it out of position for realtime data relay.

But on July 24, a rocket firing was carried out that moved the spacecraft six minutes ahead in its orbit. That should enable it to beam back telemetry during most of Curiosity's descent as originally planned.

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"Odyssey has been working at Mars longer than any other spacecraft," Gaylon McSmith, Mars Odyssey project manager, said in a NASA statement. "So it is appropriate that it has a special role in supporting the newest arrival."

MRO will record telemetry throughout the descent and play it back several hours later, after processing. MRO also will attempt to snap a picture of the MSL descent stage after parachute deploy.

The Mars Express will record most of the descent and then turn back toward Earth to relay the stored telemetry to European flight controllers. They will quickly pass it along to NASA.

But Odyssey is the key to realtime confirmation of a successful landing.

"It'll depend on how well the link is performing, what the geometry is," said Steve Sell, an entry, descent and landing engineer at the Jet Propulsion Laboratory.

"There's some uncertainty in our touchdown time just based on winds, how long we're on the parachute, atmospheric density, things like that can actually spread our landing time by about plus or minus a minute or so," he said. "So depending on exactly when we touch down determines how long we actually keep that Odyssey link active."

As a backup, the Mars Science Lab also will transmit simple tones directly back to Earth during the descent that will check off major events. But Earth will drop below the horizon as viewed from Curiosity well before landing, cutting off direct line-of-sight communications.

About the author

Bill Harwood has been covering the U.S. space program full-time since 1984, first as Cape Canaveral bureau chief for United Press International and now as a consultant for CBS News. He has covered more than 125 shuttle missions, every interplanetary flight since Voyager 2's flyby of Neptune, and scores of commercial and military launches. Based at the Kennedy Space Center in Florida, Harwood is a devoted amateur astronomer and co-author of "Comm Check: The Final Flight of Shuttle Columbia." You can follow his frequent status updates at the CBS News Space page.
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